This invention relates generally to industrial power turbines, and, more specifically, to low NOx combustors therein.
Industrial gas turbine engines may be used alone for producing power by rotating an electrical generator, for example, or may be used in a combined cycle with a steam turbine. Industrial gas turbines are continually being developed for increasing thermal efficiency, increasing specific work, reducing exhaust emissions, and reducing overhead and running costs associated therewith.
Higher thermal efficiency may be effected by increasing the firing temperature of the combustor. However, the higher combustion gas temperature increases the difficulty of reducing NOx (nitrogen oxide) emissions therefrom. NOx is one of several undesirable exhaust emissions, also including unburned hydrocarbons and carbon monoxide, which are reduced by various means.
Axially staged combustion is one method for reducing undesirable exhaust emissions while increasing the firing temperature. NOx emissions can be reduced by this method when compared to a single stage combustor. Axial staging is effected by providing fuel injection at several axial locations in a combustor correspondingly configured for this purpose. Primary fuel and air is injected at the upstream or dome end of the combustor in a first stage. As required, for meeting high power operation, additional or secondary fuel and air are injected at an axially downstream location to provide axially staged combustion.
An axially staged combustor provides low NOx operation in a dry configuration without the complexity of steam injection used in past generation industrial power turbines. However, the second stage, or secondary, fuel injectors required in axial staged combustion are necessarily located downstream in the combustor and are subject to heating by the combustion gases first generated by burning of the fuel and air mixture from the primary fuel injectors.
The secondary fuel injectors may be cooled using the fuel flow through these injectors or a portion of compressor bleed air for the injectors, but these techniques are of limited efficacy. During low power operation, the secondary fuel injectors may not be called upon to provide substantial fuel flow, so that insufficient fuel is available for cooling the secondary fuel injectors. Moreover, bleeding of compressor air for cooling the secondary injectors correspondingly decreases the overall efficiency of the engine.
Accordingly, it would be desirable to provide improved cooling of a secondary fuel injector in a dry, low NOx, axially staged combustor.